Photolithographic masking and etching is a fundamental process in the fabrication of semiconductor devices such as integrated circuits. A semiconductor wafer to be fabricated is coated with a thin layer of photoresist material and is exposed to actinic light through a patterned photomask. After development, the remaining photoresist film acts as a mask to permit selective processing, such as selective doping or etching, of the wafer.
The development of denser and more complex integrated circuits has placed increasingly higher demands on the image resolution capabilities of the photoresist film. One method for increasing resolution is known as image reversal processing which is described, for example, in the publications
(a) "The Production of a Negative Image in Positive Photoresist," S. MacDonald, R. Miller, C. Willson, Proceedings of the 1982 Kodak Microelectronics Seminar, San Diego, Calif.;
(b) "Optimization of Image Reversal of Positive Photoresist," C. Hartglass, Proceedings of the 1985 Microelectronics Seminar, San Diego, Calif.;
(c) "Image Reversal of Positive Photoresist: A New Tool for Advancing Integrated Circuit Fabrication," E. Alling and C. Stauffer, SPIE Advances in Resist Technology and Processing II, Vol. 539, pp. 195-218 (1985); and
(d) "Positive Photoresist Enhancement Options," P. Burggraff, Semiconductor International, April, 1987, pp. 84-91, at pp. 88-89.
With this process, a substrate coated with photoresist film is selectively exposed to actinic light through a mask and is then baked in an oven in the presence of amine gas. The baking step causes a chemical reaction in the portion of the film that had been exposed to actinic light, which causes it to be insoluble in a base solution as would normally be the case. After the baking step, the film is exposed to a flood of actinic light, which affects only the previously unexposed portion. Thereafter, the film is developed by selectively dissolving in a base solution the portion of the film that had originally been masked. This creates a photographic "negative" of the original image rather than a "positive" and it can be shown that, all other things being equal, the resolution of such image can be made to be sharper than that by conventional photoresist processing.
One drawback of image reversal processing is that the baking step somewhat desensitizes the unexposed film portion, which may reduce the resolution that could otherwise be obtained. Furthermore, the extent of such desensitization, and the extent of the chemical reaction in the exposed portion, typically vary from one batch to another so that the improved resolution results are not dependably predictable or reproducible.